1 Surface steam sterilization: Steam penetration in narrow channels PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de rector magnificus prof.dr.ir. C.J. van Duijn, voor een commissie aangewezen door het College voor Promoties in het openbaar te verdedigen op maandag 7 oktober 2013 om 16:00 uur door Josephus Paulus Clemens Maria van Doornmalen - Gomez Hoyos geboren te Breda
2 Dit proefschrift is goedgekeurd door de promotoren en de samenstelling van de promotiecommissie is als volgt: voorzitter: prof.dr.ir. G.M.W. Kroesen 1 e promotor: prof.dr.ir. K. Kopinga copromotor(en): dr.ir. J. van Dijk leden: prof.dr.med. A. Kramer (Ernst-Moritz-Arndt-Universität, Greifswald) prof.dr. A. Voss (RUN) prof.dr.ir. F.N. van de Vosse prof.dr.ir. A. Hirschberg (UT) adviseur(s): dr.ir. C.M.M. Luijten (ASML) CIP-DATA LIBRARY EINDHOVEN UNIVERSITY OF TECHNOLOGY van Doornmalen - Gomez Hoyos, Josephus Paulus Clemens Maria Surface steam sterilization: steam penetration in narrow channels Josephus Paulus Clemens Maria van Doornmalen - Gomez Hoyos. - Eindhoven: Eindhoven University of Technology, Proefschrift. A catalogue record is available from the Eindhoven University of Technology Library ISBN: NUR 924 Trefwoorden: stoomsterilisatie / stoompenetratie / warmteoverdracht / niet condenseerbare gassen /condensatie / numerieke modellen Subject headings: steam sterilization / steam penetration /heat transfer / non-condensible gases/condensation / numerical models Cover design: Patricia van Doornmalen - Gomez Hoyos en Paul Verspaget Printed by: Printservice, Technische Universiteit Eindhoven Part of the work described in this thesis has been carried out in the group Transport in Permeable Media at the Eindhoven University of Technology, Department of Applied Physics.
3 Aan mijn ouders
5 Contents 1 Introduction Background Current status of surface steam sterilization Outline of this thesis Steam sterilization Surface steam sterilization conditions Steam quality Steam sterilizer Surface steam sterilization process Standards A validation survey Introduction Materials and methods Steam Sterilizers Validation program Measurements of the production processes of the steam sterilizers Measurements of the daily routine control tests Evaluation of the data Statistics Results Discussion Review of surface steam sterilization for validation purposes Introduction Sterilizing conditions Validation of steam sterilization processes Number of positions to be measured Temperature requirements Entrapped air Sampling rate Criteria in standards Discussion
6 vi CONTENTS 5 F-, D- and z-values Introduction Theory Results Discussion Steam penetration into narrow channels Introduction Physical model Experimental Results Discussion Measuring NCGs during steam sterilization Introduction Principle of the ETS Physical model Experimental Results Discussion FVM modeling of steam penetration Introduction Governing equations Definitions Conservation of mass Conservation of momentum Conservation of energy Boundary and initial conditions Plasimo Some preliminary results for steam Metal plate Cylindrical bar with an open channel Discussion and outlook Discussion and outlook 95 Appendices 97 A.1 Impact of studies A.2 Steam penetration test A.3 Packing of Medical Devices A.4 Central Sterile Supply Department
7 CONTENTS vii References 103 Summary 115 Samenvatting 117
8 viii CONTENTS
9 Chapter 1 Introduction 1.1 Background Infection prevention is applied in health care facilities, pharmaceutical, and food industries, to prevent patients, staff, and environment from contamination with microorganisms. Sterilization is part of infection prevention in the health care system. Its necessity became evident over time [1 5]. Already in the stone age, in Mesopotamia, Egypt, ancient Latin America and Asia, surgery with instruments was performed [6 9]. However, cleaning, disinfection and sterilization of instruments was not addressed until Antonie van Leeuwenhoek ( ) described viable organisms. Viable organisms can be defined as organisms that are alive, capable of living, developing, or germinating under favorable conditions. Pasteur ( ), and Koch ( ) recognized that (viable) microorganisms are the carriers of diseases. It was Semmelweis ( ), however, who showed the relation between hand washing and infections of patients . Lister ( ) discovered the relation between infection of patients and medical instruments. He introduced the concept of aseptic working and the use of sterile instruments . Later on, it was discovered that microorganisms are not able to travel by themselves [15, 16]. They need a carrier, such as a liquid, a person, air borne particles, or instruments to move from one location to another. After the mechanism of transport of micro-organisms was better understood, protection of staff, patients and environment became a more important issue. Nowadays, social hygiene is widely implemented, e.g., in restaurants and food industries with a Hazard Analysis Critical Control Point (HACCP) . In health care industries and facilities infection prevention became a key issue, e.g., in dental practices, hospitals and pharmaceutical industries(see appendix A.1). Decontamination has become an essential part of infection prevention. Three levels of decontamination are recognized: cleaning, disinfection and sterilization. It can be applied on floors, worktables, surgical instruments, and medicines in closed containers [18 20]. Cleaning is rinsing and washing of the visible dirt or contamination, e.g., hand washing and rinsing of surgical instruments. Items may still be contaminated after washing. Disinfection is deactivating most of the microorganisms. It can be applied on surfaces such as working tables and endoscopes that do not penetrate the human natural barriers, e.g., the skin . Disinfection can be done with a liquid disinfectant  or exposure to an elevated temperature . Also after disinfection items may still be contaminated. Generally accepted definitions for washing and disinfection are not found in the literature. Viable microorganisms may still be present after washing and disinfection. The highest level of decontamination is sterilization. Sterile is defined as free of all viable organisms [18 20], an accepted and respected definition in health care industry and facilities worldwide [23, 24]. Sterilization
10 2 Introduction became an essential step in the process of producing sterile medical devices [15, 25]. The term medical device has a broad definition (see inset on page 2). In this thesis, unless otherwise indicated, a medical device is limited to items that are steam sterilized, such as surgical instruments. Definition of a Medical Device The definition of a medical device as given in the Medical Device Directive (93/42/EEC): medical device means any instrument, apparatus, appliance, software, material or other article, whether used alone or in combination, including the software intended by its manufacturer to be used specifically for diagnostic and/or therapeutic purposes and necessary for its proper application, intended by the manufacturer to be used for human beings for the purpose of: diagnosis, prevention, monitoring, treatment or alleviation of disease, diagnosis, monitoring, treatment, alleviation of or compensation for an injury or handicap, investigation, replacement or modification of the anatomy or of a physiological process, control of conception, and which does not achieve its principal intended action in or on the human body by pharmacological, immunological or metabolic means, but which may be assisted in its function by such means. In health care facilities steam sterilization is the most frequently applied sterilization method for reusable medical devices. Such devices may vary from textiles used during surgery to complex surgical instruments. All surfaces of these medical devices that can come into contact with the environment have to be sterile. For textiles this means all surfaces of the individual fibers, and for instruments all inner- and outer-surfaces that may come into contact with the environment. Therefore this method of sterilization is referred to as surface sterilization. In practically every hospital and in many dental and general practitioner offices worldwide steam sterilizers are used for surface sterilization of medical devices. Steam sterilizers are derived from domestic food cookers, invented in 1679 by Denis Papin ( ). Chamberlain ( ), working with Louis Pasteur, was the first to use elevated pressure for sterilization purposes (1879). Arrhenius ( ) presented the first models of thermally activated processes in the 1920s. His Arrhenius law is still in use as a basis for calculations of the killing rate of organisms in food industry, pharmaceutical industry, and health care. Until about the 1980s, various studies on killing mechanisms of organisms and sterilization have been published [26 30]. Especially in the late 1950s and beginning of the 1960s there has been a lot of activity in this field of research in the UK [31 37]. After this period the number of publications on steam sterilization was decreasing, possibly because steam sterilization was sufficiently specified for the items to be sterilized. At that time, the bulk of the items
11 1.2 Current status of surface steam sterilization 3 to be sterilized in hospitals were textiles, whereas the medical instruments did hardly change. Nowadays, textiles are being replaced by disposable solutions and, consequently, their use in hospitals reduces. Since 1990, Minimally Invasive Surgery (MIS) or laparoscopic surgery is developing rapidly. MIS has the advantage that the surgical intervention on the patient is less severe than with open surgery, resulting in a reduction of recovery and healing time and a decrease of the discomfort for a patient [38 40]. An economical side effect is a reduction of costs for patients, hospitals and insurance companies [41 43]. Instruments used for MIS may contain narrow hollow channels, i.e., channels with a radius in the order 1 mm and lengths in the order of 1 m. Establishing surface steam sterilization conditions on the inner surfaces of such channels requires that the air that is initially present in the channel is replaced by steam . This replacement appears to be far from trivial during steam sterilization, as will be demonstrated in the chapters 6, 7 and 8 of this thesis. Steam sterilization appears to be relatively safe, fast, well accepted by public opinion, and economically interesting compared to alternative sterilization methods (see chapter 2). Nevertheless, changes in items to be sterilized and the possibilities created by the application of modern techniques are not always satisfactorily addressed or researched [44 47]. In this respect we mention the necessary physical conditions and parameter measurements and steam penetration in narrow channels. In this thesis several of these points will be addressed. Unnecessarily long sterilization times Exposure times for steam sterilization are specified , e.g. 134 C for 3 minutes. In this temperature-time combination for sterilization with saturated steam, safety margins are already included. Nevertheless, frequently exposure times longer than 3 minutes are applied, up to even 7 minutes or more. Exposure of medical devices to sterilization conditions for such a long time promotes unnecessary wear out of these devices and involves excessive use of energy and water. Consequently, unnecessary costs may be initiated. 1.2 Current status of surface steam sterilization To achieve minimal acceptable levels of infection prevention in health care facilities and industry, standards and legislation are developed. This is done on a national, European, and worldwide scale. Although standards are often interpreted as state of the art, they merely address minimum requirements. Unfortunately, not all standards or legislation for steam sterilization are scientifically or evidence based [48 50]. Where possible and available, biological, chemical, or physical data are used to develop the standards. However, when insufficient data is available within the standardization committees, one tries to achieve a consensus between the participating members. Such a consensus can be based on definitions, opinions and discussions. Standards for surface steam sterilization are no exception to this procedure [51 53]. Evidently, this may introduce a false sense of safety or even unsafe situations for patients, staff and environment. Also it may result in unnecessary costs, for example, extra treatment of infected staff and patients and cleaning
12 4 Introduction costs for contaminated environment. On the other hand, it may give rise to deterioration of medical devices and energy costs resulting from unnecessarily long exposure times of these devices to elevated temperatures (see inset pages 3 and 4). Prions Diseases such as Creutzfeld Jacob (CJD) and variant Creutzfeldt-Jakob (vcjd) are related to prions. In the literature prions are described as wrongly folded proteins . Because prions, like proteins, are not viable organisms, they are not included in the definition of sterile [18 20, 23, 24]. However, like toxic matter, prions on instruments may harm people and should not be present on medical instruments used on patients. In the literature we have not found conclusive evidence that prions are made harmless in a steam sterilization process. Regardless the above, in several health care facilities steam sterilization processes e.g., 121 C for 30 minutes or 134 C for 18 minutes , are applied on medical devices which are possibly contaminated with prions. This may introduce a false sense of safety. A better solution might be to remove prions from medical instruments before sterilization, like done with toxic matter. Another option might be to adjust the current definition for sterile and include not only viable organisms but also harmful matter. In the literature and standards minimum requirements for steam sterilization are specified for surface steam sterilization [31, 51]. These time-temperature combinations are specified as minimum requirements under the assumption that saturated steam is present on all surfaces to be sterilized. In these time-temperature combinations the temperature is assumed to be constant. Often aqueous medicines disintegrate at elevated temperatures and the specified time-temperature [56 58]. In these cases the so-called F-value theory may be used to limit the exposure time to elevated temperatures but also to optimize the sterilization process. This theory comprises a mathematical model to calculate the equivalent of the time-temperature combination of an accepted sterilization process [18, 59]. Basically this calculation is an integration of the killing of organisms over time. The F-value is calculated from the moment that sterilization conditions are present, e.g., at temperatures of 105 C and higher. For example, an accepted and standard time-temperature combination for aqueous medicines in closed ampules is 120 C for 20 minutes. In this case the reference value is F 120 C 20min and is called the F 0 -value. With the mathematical model the F-value of an actual process is calculated from behavior of the temperature as function of the time. The calculated F-value should be equal or larger than the F 120 C 20min-value (20 minutes). Currently a similar method is used to calculate the disinfection period in washer-disinfectors in hospitals [22, 60, 61] and is called the A 0 -concept or A-value method. Unfortunately, it is not documented in the literature on which temperature domain the F-, and A-values can be applied, because in these methods the killing rate of organisms is linearized around a certain reference temperature. In chapter 5 the F-value theory is discussed in more detail. It is shown that the currently used methods can be extended in a straightforward way to the entire temperature region of interest. Although it is difficult to prove that insufficient sterilization may cause infection, contaminations of patients by instruments have been reported in the literature [1 5, 15, 62
13 1.2 Current status of surface steam sterilization 5 68]. Also it has to be noted that the incubation time before symptoms of a contamination show up makes it difficult to identify the contamination source. This may lead to unnecessary discomfort and costs (see inset page 5). If a contamination occurs, patients generally consult a General Practitioner (GP) for treatment without knowing or identifying the source where they were contaminated. This illustrates also that the relation between sterilization and infection prevention is often difficult to quantify. Possibly, not all incidents are published because their cause was not identified, or they are undocumented because of privacy and legislation, or for less ethical reasons. Fortunately, a tendency is noticed that health care facilities are implementing patient surveillance systems and systems to monitor (track and trace) medical devices. In the patient file the medical devices used on the patients are registered, making it easier to recognize relations between infections of patients, used medical devices and sterilized batches of these devices. An additional economical advantage for the health care facility is that these monitoring systems can be used to manage, control and schedule preventive maintenance of medical devices and the equipment used for decontamination. Choice of an effective steam sterilization process A steam sterilization process suitable for non wrapped, solid instruments, is called a type N process . A sterilizer equipped with N processes costs about AC 2,000.. A steam sterilization process to sterilize wrapped porous loads is called a type B process . Steam sterilizers equipped with these processes cost about AC 4,000., roughly AC 2,000. more. In dentist treatments hollow instruments are often used, e.g., hollow drills for implantology and re-usable multi-function syringe tips . According to standards a type B processes should be used for this type of devices. If a type N process would be used to sterilize these hollow devices a patient may be contaminated during a dentist treatment. Not taking the discomfort for the patient into account, the costs for treatment are at leastac 30. ;AC 20. for the consultation of a dentist or general practitioner, and AC 10. for the medicines. This hollow instrument may remain contaminated and form a source of contamination and risk for patients. Again not taking the discomfort into account, after 70 contaminations with this device or other devices a sterilizer with a type B process would be profitable. One should note that oral herpes is a relatively harmless infection compared to for example a hepatitis infection. Obviously, the return of investment for more severe infections will be much faster and the discomfort for the patients will be substantially reduced. Because the contaminated patients usually go to a general practitioner for treatment and not to the actual place of the contamination, a patient surveillance system could make this issue better visible for patients, dentists and insurance companies. If surface steam sterilization is applied, it should be done in an effective and reproducible way. In this thesis, effective means that all surfaces are exposed to steam sterilization conditions for a predetermined time. With reproducible is meant that each time a sterilization cycle is executed, the conditions on all surfaces are similar to the runs before. When steam faces barriers, establishing steam sterilization conditions on surfaces becomes more difficult. Barriers can be porous loads, the wrapping of instruments to be sterilized, or instruments with cavities, such as devices with narrow channels. For these
14 6 Introduction items replacement of air by steam may require additional attention . It is remarkable that criteria for steam sterilization differ depending on the geographical position, whereas the aim is the same. As an example we mention the performance requirements of the steam penetration test(appendix A.2). The requirements for such a test in Europe  and the USA  differ essentially, whereas the objective, production of sterile medical devices, is the same. It is also remarkable that criteria [51, 52] for large and small steam sterilizers differ, although in both types of sterilizers similar items may be sterilized . It is likely that microorganisms behave similar all over the world, and therefore procedures, processes and standards for sterilization should be similar worldwide as well. Summarizing, with a better understanding of surface steam sterilization, suboptimal processes can be optimized, resulting in an improvement of steam sterilization processes, global differences in criteria can be reduced, and a false sense of safety can be avoided. More important, effective sterilization introduces less infections of patients, staff and environment, and decreases costs. 1.3 Outline of this thesis This thesis aims to contribute to the fundamental understanding of surface steam sterilization, steam sterilization processes and penetration of steam in medical instruments with narrow channels during such processes. In chapter 2 the basic concepts of surface steam sterilization, steam sterilizers and sterilization processes are reviewed. This chapter also briefly addresses the current standards for steam sterilization. It is followed by chapter 3, a survey on the validation status of 197 steam sterilizers in Dutch hospitals, in 2001 and The results of this survey showed that only 40% of the hospitals in this survey did fulfill the claims they made with respect to their steam sterilization. All these claims were made based on standards. This initiated the study reported in chapter 4, in which we investigated to which extent these steam sterilization standards cover steam sterilization conditions as specified in the literature. The study shows that monitoring and validation of steam sterilization processes based on temperature and pressure measurements 1 is only valid in specific situations. In the literature an alternative method for monitoring, the F-value theory, is described. In chapter 5 the limitations of the original F-value theory are discussed and an improved model is proposed. This modified model can be applied over a broader temperature range. However, even if the sterilization conditions are satisfied within the sterilizer chamber itself, this does not necessarily imply that all types of loads can be properly sterilized. For instance, with the development of MIS instruments more surgical instruments contain hollow narrow channels. Steam penetration in these channels appears to be far from trivial. In chapter 6 a model for steam penetration in narrow channels in the absence of condensation is discussed, where special attention is given to the effect of non-condensible gases (NCGs). In chapter 7 experiments are presented that were performed to quantify the 1 In this method a so-called theoretical temperature is calculated from the measured pressure . In the standards  criteria are given to which extent the measured temperatures and this theoretical temperature should agree.
15 1.3 Outline of this thesis 7 sensitivity to NCGs of a commercially available instrument to assess steam penetration, together with a theoretical model to explain these experimental results. In this model condensation is assumed to be dominant. Both models in chapters 6 and 7 are quasi onedimensional and quasi isothermal. In chapter 8 a two dimensional and non-isothermal theoretical model is discussed. In the last chapter 9 the conclusions, discussions, and an outlook are given. Chapters 3, 4, 5 and 6 of this thesis are papers that have been published or submitted, which are included in their original form. Consequently, some overlap between parts of these chapters is unavoidable.
16 8 Introduction
17 Chapter 2 Steam sterilization Free of all viable organisms is worldwide accepted as the definition for sterility of medical devices[18 20,23,24]. Toprovethatamedical deviceisactuallysterileithastobetested. During this testing the item is handled and manipulated and cannot be considered sterile anymore. Consequently testing sterile items before use is pointless and even impossible. Therefore a statistical approach was and is necessary. Favero  described how the statistical definition of sterility originated, how it evolved and how the concept of the Sterility Assurance Level (SAL) developed over time and is applied in practice. Currently, the European standard EN 556  defines For a terminally-sterilized medical device to be designated STERILE, the theoretical probability of there being a viable microorganismpresent on/inthe device shall beequal toor less than Alargely similar concept is the SAL, which is defined as the probability of a single viable microorganism occurring in or on a product after sterilization . To meet the sterility requirement the value of the SAL should be 10 6 or less. It can be stated that worldwide the accepted statistical definition is Sterility of medical devices is defined as the chance of finding a viable organism in or on a medical device being 1 in 1,000,000 or less. Statistical definition of sterile In principle, the statistical definition of sterile  could be interpreted as that at most 1 of 1,000,000 sterilized items may contain a viable organism . However, it is obvious that the surface of, for instance, 10 6 scalpels is much smaller than that of 10 6 orthopedic drills. Therefore this definition contains a sliding scale with respect to the actual surface area. Apart from this, there is an ongoing discussion whether the actual sterilization process should always reduce the amount of viable organisms by a factor of 10 6 [46, 47]. When the definition free of viable organisms is accepted and is applicable to the medical devices, all steps to produce a sterile item should be considered. In hospitals, these steps often include washing, disinfection and sterilization. Each step leads to a certain reduction of the amount of viable organisms. If before sterilization an item would already be free of viable organisms, it might be considered sterile. However, it is not wrapped and can get recontaminated during handling, transport and storage (see appendix A.3). Nevertheless, if the initial contamination before the actual sterilization process is known, that process might be adjusted accordingly. Standards for sterilization allow for such an approach . Often sterilization is associated with inactivation of organisms rather than removing . Although viable organisms can be separated from a fluid by filtration, this is often not considered as sterilization. Filters can start leaking and filtration does not kill organisms. Exposing organisms to deadly conditions will inactivate or destruct them. A
18 10 Steam sterilization possible classification of different sterilization methods can be based on the temperature: low temperature sterilization, i.e., sterilization at temperatures below 100 C, and high temperature sterilization, at temperatures above 100 C. Examples of low temperature sterilization are ethylene oxide sterilization, formaldehyde-, hydrogen peroxide-, plasma-, ozone-, and irradiation sterilization. Examples of high temperature sterilization are dry heat and steam sterilization. Another basis for classification of sterilization methods could be the mechanism of killing the viable microorganisms, such as oxidation, intoxication, destroying vital cell structures. Oxidizing sterilization methods 1 are hydrogen peroxide, ozone, and dry heat sterilization, intoxicating methods 2 are ethylene oxide and formaldehyde, and examples of sterilization methods based on changing vital cell structures 3 are irradiation and steam sterilization. Steam sterilization in dental offices Dry heat sterilization temperatures are typically above 150 C. At a specified temperature, exposure times are specified to produce sterile items, e.g., at 160 C the exposure time is 2 hours (see table 2.2). The complete process cycle of a dry heat sterilization process, with warming up and cooling down, will take over 3 hours. A typical exposure time for items processed in steam sterilization at 134 C is 3 minutes. A complete steam sterilization cycle will be ended in about 1 hour. Energy costs of dry heat sterilization are higher than those of steam sterilization. In addition, MIS and complex instruments, e.g., hand-pieces for dentistry, are often made of different parts, consisting of different materials with different thermal expansion characteristics. Most plastics and polymers used in a medical device will deform irreversibly at temperature above 140 C. In a medical device different materials may be welded together, or moving within each other, e.g., the turbine to drive the drill in a dentist hand-piece. The higher the temperature, the bigger the difference in expansion of the various materials, and the faster the wearing out of the expensive medical device. Obviously, compared to dry heat sterilization, steam sterilization will safe time, energy, wearing out of instruments, and therefore costs in dental practices. It is important to realize that sterilization is only possible if the organisms are in contact with the sterilization agent or sterilant. However, as mentioned above, sterilized items should not be touched or handled anymore after sterilization. Consequently, devices have to be protectively wrapped before sterilization to prevent re-contamination after sterilization. Protection can be done by wrapping the devices before sterilization in a micro biological barrier, for instance sheets of crepe, or by packing in a container (see appendix A.3). Obviously the sterilant must be able to penetrate through the wrapping. Every sterilization method has its advantages and disadvantages. Nevertheless, all sterilization methods have the ability to kill and are therefore by definition hazardous. A sterilant can be even classified carcinogenic, e.g., formaldehyde . This concerns not 1 Sterilization methods in which oxygen out of the environment is used to burn the viable organisms which are present. 2 Sterilization methods in which the viable organisms are poisoned. 3 Sterilization methods which irreversible change the vital cell structures necessary for life, e.g., DNA strings.
19 2.1 Surface steam sterilization conditions 11 only the target group but also the operators of a sterilizer and its environment. Depending on the specific situation the most effective and safest sterilization method should be chosen (see inset page 10). In pharmaceutical and food industries, for example, items are sterilized only once before being transported to the end-user. During the transport from industry to end-user heavy wrapping is necessary to prevent re-contamination but also to protect the devices and their micro biological barriers from damaging. Consequently, sterilization methods in industries have to be able to penetrate the heavy transport wrapping in order to come into contact with viable microorganisms. Irradiation and ethylene oxide have good penetration capabilities and are more often applied in industries than in health care facilities. Industrial sterilization is performed at the end of a production line and no or only limited reprocessing is performed. On the other hand, in health care facilities, e.g., hospitals and dental practices, sterilization of re-usable medical devices such as surgical instruments and hand-pieces is performed frequently. Reprocessing of re-usable medical devices can be classified as an expertise. To perform the reprocessing and sterilizing of medical devices efficiently and by experts, a so called Central Sterile Supply Department (CSSD, appendix A.4) can be found in the larger health care facilities. In health care facilities worldwide steam sterilization is the most generally applied sterilization method for various reasons. First, the bulk of reusable medical devices can be processed in it. Second, compared to other sterilization methods it is the least hazardous for staff and environment and is therefore socially accepted. Third, the working mechanism of steam sterilization is fairly well understood and described in the literature [18 20]. Fourth, alternative methods demand extra safety requirements [75, 76]. These extra requirements can be hardware, such as sensors, and monitoring systems for locations and staff, written procedures for operating the machine, including safety and calamity procedures, and even requirements for specialized staff to operate the sterilizer. Finally, steam sterilization has economical advantages compared to other sterilization methods. The result of a sterilization process depends on the combination of the sterilizer, process, load, loading pattern (placement of the instruments in the sterilizer) and wrapping. In this chapter the surface steam sterilization conditions (section 2.1) and steam quality (section 2.2) will be addressed, followed by the explanation of the working principle of a steam sterilizer (section 2.3) and steam sterilization processes (section 2.4). Over time standards for steam sterilization have been developed and published [23, 51, 53, 70, 71, 77 80]. In section 2.5 these will be reviewed briefly. 2.1 Surface steam sterilization conditions Steam sterilization is generally applied in two ways: sterilization of aqueous liquids in closed containers and surface steam sterilization. Regardless the method the killing mechanism is coagulation . Without the proteins organisms are not viable and cannot live. Coagulation requires energy and water [20, 28, 81]. Before coagulation can take place the protein string has to be broken up into smaller chains. In steam sterilization processes for aqueous liquids in closed containers the steam is used to heat up the liquid in the container. By heating up the aqueous liquid hydro-sulphide ions and smaller peptide chains
20 12 Steam sterilization may be detached from the proteins [81, 82]. Like water molecules these hydro-sulphide ions and peptide chains are bipolar. Therefore, these detached ions and chains can move through the water to new other locations within the organism. At these locations new bonds between the molecules are formed, the actual coagulation. These new bonds are different from the original bonds and are generally harder and irreversible. Because these newly formed molecules differ from the proteins chains, life and viable life is destroyed [20, 28, 81, 82]. In this thesis coagulation is defined as the irreversible change and hardening of the protein chains of a microorganism. In aqueous environments the water content of cell structures is optimal for sterilization. However, contamination of medical instruments occurs on the surfaces of these devices. Before being exposed to surface steam sterilization, the amount of water in the viable organisms on these surfaces depends on the type of organism and the environmental conditions [26, 83]. If only exposed to elevated temperatures with insufficient water in the organism, ions and peptide chains may not be detached and able to relocate. However, the steam is not only supplying the energy for increasing the temperature. Steam condenses on colder surfaces and provides a condense layer. This layer establishes the necessary wet environment to transport the ions and peptides to other locations to form new irreversible bonds. Chaufour et al.  showed that items have to be cleaned before surface sterilization. A layer of organic material or dirt can prohibit the creation of sterilization conditions. Apart from this, when biologically incompatible materials such as endotoxines are introduced in a patient, the patient may develop an infection. In aqueous steam sterilization the amount of incompatible materials is often controlled with aseptic processes. However, onasurface ofamedical device theamount ofsuch materialshastobereduced asmuch as reasonably achievable before sterilization. Cleanliness of surfaces after cleaning processes is not trivial, especially not for the inner surfaces of channels in medical devices [85 87]. In such cases sterilization is the final security that no viable organisms are brought into a patient. Sterilization can be defined as establishing sterilization conditions and maintaining these for a predetermined time. Surface steam sterilization conditions are specified as sterilization of clean surfaces with saturated steam at a predetermined temperature. The only time-temperature combinations for surface steam sterilization with saturated steam found in the literature are those of the Working Party on Pressure-Steam Sterilizers of the Medical Research Council . In table 2.1 these combinations are presented together with the time-temperature relations determined for sterilization of aqueous liquids by Perkins . 2.2 Steam quality In steam sterilization of aqueous liquids in containers the steam does not come into direct contact with the liquid. As long as the liquid reaches the predetermined temperature sterilization will occur. For surface steam sterilization the quality of the steam is more important, because the steam will be in contact with the surfaces that have to be sterilized. In steam sterilization two aspects of steam quality can be distinguished. First the